In a conventional vehicle, an internal combustion engine generates rotary power by burning fuel, and the power is transmitted to the driving wheels through a mechanical transmission. The engine is the only power supply to the vehicle and has to provide all the torque that the vehicle needs. When cruising on a flat road, the vehicle only needs relatively small power or torque to maintain its speed, but it takes much more power or torque to accelerate or go uphill. The engine has to be able to provide the torque required for acceleration and uphill, so the engine must be very powerful. Usually a bigger engine loss more energy due to friction than a smaller one. On the other hand, high power is needed only for a small fraction of working time, so for most of the time, a significantly small engine will work well for the vehicle, and it can save a lot of energy. When the vehicle applies braking, the kinetic energy of the vehicle is converted into heat, and a lot of energy is lost.
Also, when a conventional vehicle stops at traffic light, the engine keeps running, burning out fuel unnecessarily.
In order to improve fuel efficiency and reduce emission, hybrid electric vehicles are proposed. One or more electrical motors are used to assist the engine when high torque is needed, so a smaller engine may work well. The motor(s) also can re-generate electric power while applying brake torque on the wheels. An electric energy storage device is employed to store electric energy when it is available and to provide energy to the motor(s) when needed.
One of hybrid electric drive systems proposed has an electric motor between the engine and the transmission, and the motor shaft (rotor shaft) is permanently connected to the crankshaft of the engine. During startup of the vehicle, the motor functions as a starter, rotating the crankshaft of the engine while the cylinders are fired. After the engine is started, the motor is used as a generator to maintain the electrical system of the vehicle. The motor can assume other functions such as stopping and restarting the engine to save fuel during idle and deceleration, aiding the engine during heavy accelerations and capturing energy during decelerations.
One drawback to such a system is that the motor shaft is fixedly attached to the engine shaft. When the engine is not operating, it may be desirable to provide the transmission with power supplied by the electric motor. However, to do this the crankshaft of the engine is rotated. This is inefficient because an increased amount of power must be provided.
To solve this problem, a system is proposed in U.S. Pat. No. 6,364,042. It comprises an engine, an electric motor (starter/generator), a transmission, and a clutch. The motor shaft is connected to the transmission input shaft, and the clutch selectively couples the motor shaft and the engine shaft. In the case that it is desirable to run solely on electric power, the clutch is used to disengage the engine shaft from the motor shaft. This allows the motor to operate as a motor to power the vehicle without having to rotate the engine shaft. This mode of operation is useful at low speeds such as in city driving. When desired, the system controller can control the system to operate with the engine, and the motor can operate as an alternator to charge the batteries.
One drawback to such a system is that: it takes many steps and a long time to start the engine while the vehicle is running. Here are the steps: Step 1, disengage the main clutch in the transmission to disconnect the motor shaft from the transmission shaft. Step 2, slow down the motor to zero speed. Step 3, engage the clutch and connect the engine shaft to the motor shaft. Step 4, speed up the engine shaft and start the engine when it reaches its idle speed. Step 5, engage the main clutch in the transmission and connect the engine to the transmission input shaft. It would take up to a couple of seconds to go through all the steps, and the vehicle does not have power meanwhile. This is a significant issue for the vehicle's performance.
The purpose of this invention is to provide a hybrid electric drive system that can start the engine quickly and smoothly and does not need a complicated control system.